Ultrafast optical coincidence detector which utilizes the (1,-1,0) surface or its crystallographic equivalent in crystals of the 42m or 43m class for mixing two orthogonally polarized pulses



4 "3 5-508 A J 2 3 3 EX 5 I D 8 1 Oh X R 3 Q 4 "i5 6b 3 May 20, 19693,445,668 ULTRAFAST OPTICAL COINCIDENCE DETECTOR WHICH UTILIZES THE (1,-1, 0) SURFACE OR ITS CRYSTALLOGRAPHIC EQUIVALENT IN CRYSTALS OF THE 42mOR 15m CLASS FOR MIXING TWO ORTHOGONALLY POLARIZED PULSES Filed May 4,1967 J A. ARMSTRONG 1 DELAY- M //T\ PULSE mom 4L DELAY N I PULSEWIDTHLASER PULSES PHOTOMULTIPLIER ARRIVING AT CRYSTAL IGNA SURFACE s L 21FIG. 4

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INVENTOR JOHN A. ARMSTRONG BYWZ ATTORNEY United States Patent 3,445,668ULTRAFAST OPTICAL COINCIDENCE DETECTOR WHICH UTILIZES THE (1, -1, 0)SURFACE OR ITS CRYSTALLOGRAPHIC EQUIVALENT IN CRYSTALS OF THE 12m OR 13mCLASS FOR MIXING TWO ORTHOGONALLY POLARIZED PULSES John A. Armstrong,South Salem, N.Y., assignor to International Business MachinesCorporation, Armonk, N.Y., a corporation of New York Filed May 4, 1967,Ser. No. 636,105 Int. Cl. H01j 39/12; G02f 1/28 US. Cl. 250-217 6 ClaimsABSTRACT OF THE DISCLOSURE An optical pulse coincidence detector whosecoincidence resolving time is faster than lfl 'seconds. The detectorutilizes the optical sum-frequency (of two coincident pulses) generatedin reflection at the surface of a single crystal of GaAs, which opticalsum-frequency is generated only when the two coincident pulses areorthogonally polarized. The optical sum-frequency is a particular colorand the detector can be made sensitive to this color.

Background of the invention Methods for mode-locking lasers employingNd-doped glass or Nd-doped yttrium-aluminum-garnet as active media inlasers have been achieved, and such lasers are described in articlesappearing in the 1966 issue of Applied Physical Letters 8, page 174+ andpage 180+. Such mode locking results in the production of extremelyshort pulses, i.e., of the order of 10- seconds in duration. Measuringand detecting equipment for such ultrashort pulses are not available,resulting in an inability to determine some of the basic characteristicsof such pulses.

The present invention provides an optical pulse coincidence detector asa means for sensing ultrashort optical pulses. In a coincidence detectoran output pulse is produced only if two inputs arrive simultaneously atthe detecting unit. The detector employs the special symmetry propertiesof optical sum-frequency generation in reflection at the surface of aGaAs single crystal. Although GaAs is very nonlinear optically, thereare a number of orientations of the polarization of an incident lightpulse such that no harmonic or sum-frequency light will be generated.However, if two pulses whose coincidence is to be detected have suitableorthogonal polarizations, there will be efficient sum-frequencygeneration at the crystal surface.

The above mentioned elfect can be employed to measure the widths andshapes of such ultrashort pulses. To accomplish such measurement, thefast pulse train emanating froma mode locked laser is divided into twobeams. One of the two beams is passed through a z-cut quartz element andits polarization is rotated 90, making such beam be polarizedperpendicular to the plane of incidence. The beam whose polarization hasbeen rotated 90 is made to pass through a fixed, totally reflectiveprism which directs it back to a mirror and a second beam splitter. Atthis second beam splitter, it is recombined with the beam whosepolarization has been undisturbed, the latter beam having traversed apath whose length can be varied by moving a second prism. The usefuloutput of this part of the apparatus will comprise two superposed,parallel beams of orthogonally polarized pulses having an adjustabledelay between them, the adjustable delay being provided by the movableprism. The average width of the pulses in the train is measured bymeasuring the amount of movement of the movable prism needed to destroythe coincident arrival of the two pulses at the crystal surface.

Consequently it is an object of this invention to provide a novelcoincidence detector.

Yet another object is to provide a detector circuit capable of detectingoptical pulses as short as 10- seconds or shorter.

A further object is to employ such novel detector in a system formeasuring extremely narrow optical pulse widths.

Yet another objec is to attain an accurate measurer of picosecond laserpulse widths employing relatively few components.

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings.

Brief description of the drawings FIG. 1 is a schematic representationof the novel coincidence detector.

FIG. 2 is a schematic representation of the use of such coincidencedetector in a system for measuring pulse widths of the order of 10-seconds.

FIG. 3 is a showing of harmonic light generated by GaAs as a function ofthe delay between the laser pulses.

FIGURE 4 is a plot of harmonic signal output from the detector as afunction of position of the movable prism.

Description of the preferred embodiments The detector 2 of FIG. 1comprises a single crystal of GaAs out so that any incident light to bedetected will fall on the (l, -l, 0) plane. 0 A is the direction'ofincidence of the pulses whose coincidence is to be detected. Assume thata first pulse p1 is polarized in the direction shown by the vector a andthe second pulse p2 is polarized in the direction shown by the vector b,the two vectors being at right angles to each other. The GaAs crystal isvery nonlinear optically and there are a number of orientations of thepolarization of an incident light pulse (including directions a or babove) for which no harmonic or sum-frequency light will be generated.However, when two coincident pulses are orthogonally polarized, there ishighly efficient sum-frequency generation at the crystal surface 4during the overlap of such orthogonally polarized pulses.

The sum-frequency is generated only along the direction KY3, where DAB=0AD, and AT is the normal to crystal surface 4. The sum-frequency beamis polarized in the direction along the vector c. A detector 6, such asa photomultiplier, is placed in the path that intercepts sum-frequencybeam XE. As an additional aid in obtaining selectivity in detection, afilter 8 is employed between crystal surface 4 and photomultiplier 6 totransmit only the sum-frequency light to photomultiplier 6, attenuatingany reflected laser light from surface 4.

Although the coincidence detector has been shown to be a GaAs crystalhaving a detecting surface lying in the (1, l, 0) plane and the [001]crystal direction is normal to the plane of incidence of the light to bedetected, the invention can be generalized in several ways. Any crystalhaving the same symmetry as GaAs, namely, 13m, or any crystal of thesymmetry of potassium dihydrogen phosphate (KDP), namely, 32m, could beused in place of and in the same configuration as GaAs shown in FIG. 1.A more detailed description of classes l2rn and 33m is found in a bookentitled Introduction to Solid State Physics--Kittel, page 17+,published by Wiley & Sons, 2nd edition, 1956, New York, that sets out ashort form of the International Symbols for the Crystallographic PointGroups for identifying the symmetry class of crystals.

The invention also embraces the modification that includes rotating thecrystal of FIG. 1 by 90 around the normal E to surface 4. Accompanyingthis rotation,-

the polarizer 8 is also rotated 90 so that the latter will transmitsum-frequency light polarized perpendicular to the plane of incidence.Moreover, it is well within the contemplation of this invention that thesum-frequency to be detected need not be restricted to the secondharmonic of a given incident frequency. The two pulses whose coincidenceis to be detected can be two pulses of different frequencies, i.e., (vand ta but orthogonally polarized as discussed hereinabove. The detector6 of FIG. 1 is then made sensitive to the sum-frequency w +w rather thanthe harmonic frequency of the example given to illustrate the invention.

The following equations represent components (P P P of the sum-frequencydielectric polarization produced in the crystal by the incident opticalpulses E(w and E(w The axes x, y z are chosen to correspond to the x, y,z axes of the crystal where w =w +w and Q1 is the frequency of a firstpolarized laser beam and 02 is the frequency of a second polarized laserbeam. P is the x component of the sumfrequency dielectric polarization;P and P represent the x and y components of the sum-frequencypolarization respectively; x equals the nonlinear susceptibility of amedium; E represents an electric field and E E and E are respectivelythe x, y and z components of such electric field.

The operation of the coincidence detector will be described in terms ofthe above equations. In FIG. 1, let the incident pulse p1, polarizedalong a, be at frequency and pulse p2, polarized along b, be atfrequency 40 As can be seen from the coordinate system of FIG. 1, theelectric field E(w is polarized along the z axis, i.e., E (w )=E (w)==0. On the other hand, pulse p2, when refracted into the crystal, willhave no component along the z axis, i.e., E (w )=0. However, E (w and E(w are 0. The nonlinear polarization P(w can now be evaluated explicityusing Equations 1, 2 and 2.

Assume, first, pulses p1 and p2 do not overlap during their arrival atthe detector. Then it is seen from such equations (each term to theright of equal sign involves the product of simultaneous fields at thetwo frequencies) that all components of P(w will vanish and there willbe no sum-frequency light generated at the surface 4 of the crystal.Assume alternatively that pulses p1 and p2 overlap at their arrival atcrystal surface 4, i.e., there will be simultaneous non-zero fields atA0 and 01 Thus Equation 1 becomes Equation 2 becomes P (w )=x[0X0+E (w)E (w and Equation 3 becomes Since P =0, and P and P are 0, asum-frequency signal will be radiated in the plane of incidence.

In the case where the frequencies m m are equal (harmonic generationinstead of sum-frequency genera tion), Equations 1-3 can be analyzed inan analogous manner, and it will be seen that a harmonic signalpolarized parallel to the plane of incidence can be produced only ifpulses p1 and p2 overlap in time at surface 4.

FIG. 2 is employs the detector of FIG. 1 to measure pulses that are ofthe order of picoseconds in width. A mode-locked laser emits a train ofshort pulses whose average width is to be measured. The train ispolarized parallel to the plane of the drawing by polarizer P, beforebeing split into two beams I and II by beam splitter 12. Beam I ispassed through an optically active z-cut quartz crystal 14 which rotatesits plane of polarization 90, making such beam I perpendicular to theplane of the figure and to the plane of incidence at the GaAs surface 4.Such beam I passes through a fixed, totally reflecting prism 16 whichdirects the beam back to a two-sided mirror 18. Beam II traverses thebeam splitter 12 and is refiected by mirror 18 to a movable prism 20.The two beams I and II are merged at another beam splitter 22, 24representing the discarded beams and 26 being the merged portions thatimpinge on the surface 4 of the crystal.

The two beams I and II fall on the surface 4 of single crystal of GaAs,the latter generating second harmonic light 28. In a particular type ofmode-locked laser, whose active medium was neodymium-doped glass, theemitted laser pulses had a wavelength of 1.06 t. Consequently, thesecond harmonic wavelength of beam 28 is 0.53

A filter 30 preferentially allows the passage of light having awavelength of 0.53/1. and the polarizer 32 and photomultiplier 34complete the detection scheme so that only second-harmonic lightpolarized parallel to the plane of incidence is recorded. Attenuator 29,composed of an aqueous solution of CuSO is used to attenuate the laserbeam to prevent damage of photomultiplier 34.

The GaAs crystal is oriented so that the direction is normal to itssurface, and the [001] direction, which is perpendicular to the plane ofincidence, is parallel to the polarization of beam I. From the symmetryof the nonlinear susceptibility tensor for GaAs, well known to thoseskilled in the art of nonlinear optics, it follows that with the abovenoted orientation of the crystal surface 4, neither of the linearlypolarized beams at 1.06 acting alone, will generate second harmonics ofthe required polarization. However, when the two beams I and II overlapduring their arrival at the crystal surface 4, the resultant total laserfield during the overlap has x, y, and 2 components and produces asecondharmonic reflected beam with a large component of parallelpolarization.

The amount of harmonic signal produced depends on the degree of overlapof the pulses I and II. For example, in FIG. 3, two orthogonallypolarized beams are separated by more than a pulse width so that thereis no overlap and hence no harmonic light generated at the surface 4 ofthe GaAs crystal; this is shown as the horizontal line L. By movingprism 20 closer to mirror 18, a smaller delay between pulses is producedand the pulses overlap, producing an output M at the photomultiplier 34.Where the prism 20 is moved so that the delay between the arrival of thetwo beams at crystal surface 4 is much less than the width of thepulses, a maximum output N is produced by photomultiplier 34.

The delay between beams can be varied by movable prism 20 in intervalsas small as 10- sec. In FIG. 4, the normalized harmonic signal fromcrystal surface 4 is plotted against the position of the movable prism20.

The duration of the pulses under study by my detector is calculated froman experimental curve such as that shown in FIG. 4 by the followingformula:

At=2Al/BC (4) where At is the full width in seconds at half-maximumintensity of a typical pulse, Al is the full width at halfmaximumintensity in centimeters of the experimental result shown in FIG. 4, Cis the velocity of light in cm./ sec., and B is a numerical factordependent on the shape of the fast pulse under study and varies from 2to V2. For data relied on to plot FIG. 4, AI=0.-13 cm., 8:2, and C=3 l0cm./sec., At=4 10- cm. The factor 2 in the Formula 4 reflects the factthat the delay introduced in the path of light beam II by movable prism20 increases twice as fast as the;change in the position of the prism.

In an actual experimental setup shown in FIG. 2, two other detectors areshown indotted boxes B1 and B2. A portion of the original pulse train,after passing through polarizer P, is diverted into detector B2 by beamsplitter 36 and another portion is-directed by beam splitter 38 intodetector B1, consisting of lens 40, filter 42, z-cut quartz crystal 44,attenuator 46, filter 48 and photomultiplier 50. The second harmonicsignal generated by the z-cut quartz crystal 44 is used to normalize theharmonic signal generated by GaAs crystal. Such normalization is astandard procedure in harmonic generation with pulsed lasers to protectthe experimenter from changes in characteristics of the mode-lockedlaser source. The fast pulses of the mode-locked laser are alsomonitored by attenuator 52, filter 54 and photocell 56, the output ofwhich goes to a Tektronix 51'9 oscilloscope, not shown. The purpose ofthe monitoring scope 519 is to allow one to reject data obtained byphotomultiplier 34 when such scope 519 has indicated that the originallaser source has produced a pulse train that is significantly differentfrom a standard useful pulse train.

The coincidence detector of the present invention is particularlyadvantageous in that it can detect pulses of the order of or 10" sec.duration; the output energy of such detector for coincidence is 100times greater than when pulses being detected are not in coincidence.Consequently, discrimination between wanted and unwanted signals isextremely high. Furthermore, a 0.001 inch movement of the prism resultsin a 0.002 inch increase in path length for beam H, and such 0.002 inchincrease in path length corresponds to a time interval of about 2 10-sec. (the amount of time light takes to travel 0.002 inch). The resultsof a series of such measurements give the pulse Width and shape as shownin FIG. 2b, such plot indicating a pulse width of the order of 4X10-sec. Obviously accurate measurement of such narrow pulse widths is ofgreat aid not only in the study of laser characteristics, per se, but inthe field of nonlinear optics generally.

While'the invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that the foregoing and other changes in formand details may be made therein without departing from the spirit andscope of the invention.

What is claimed is:

1. A coincidence detector comprising a crystal of the 42111 or l lirnclass, said crystal being cut along a (l, l, 0) surface or itscrystallographic equivalent to present a surface to radiation to bedetected, such surface being oriented so that the [001] type directionlying in that surface is either perpendicular to or paralle to the planeof incidence of said radiation on said surface,

means for transmitting two radiation pulses that are orthogonallypolarized with respect to each other and are either perpendicular orparallel to the plane of incidence,

said pulses impinging on said surface whereby the latter generates asum-frequency radiation that is polarized in a direction parallel to aperpendicular plane of incidence, said generation occurring only if saidpulses are coincident at said surface, and

radiation detector means in the path of said sum-frequency radiation.

2. A coincidence detector comprising a crystal of GaAs, the latter beingcut to form a surface lying in the (l, -1, 0) plane or itscrystallographic equivalent,

two simultaneous orthogonally polarized pulses impinging on saidsurface, said polarizations being either perpendicular or parallel tothe plane of incidence of said pulses,

the impingement of such orthogonally polarized pulses producing asum-frequency radiation output whose plane, of polarization is parallelto the plane of incidence, and

means for detecting such sum-frequency radiation.

3. The detector of claim 2 wherein a filter, transparent only to saidsum-frequency radiation, is interposed between said GaAs crystal andsaid sum-frequency radiation detecting means.

4. The detector of claim 2 wherein said orthogonally polarized beams arelaser beams.

5. A system for measuring ultrashort laser pulse widths comprising asingle crystal of GaAs cut so as to provide a surface in the (1, l, 0)plane or its crystallographic equivalent,

means for dividing each pulse into two pulses, the first of said twopulses being polarized orthogonally to the second of said two pulses,

means for varying the length of the path of the second pulse withrespect to said first pulse so as to provide an adjustable delay betweensaid two pulses, means for merging said two pulses,

means for impinging said orthogonally polarized pulses onto said crystalplane so as to create a sum-frequency radiation generation of said twopulses at said surface, only when such pulses are in coincidence, and

means for detecting said sum-frequency radiation.

6. A system for measuring ultrashort laser pulse widths comprising asingle crystal of the 12m or 13111 class cut so as to provide a surfacefor generating harmonic radia tion,

means for dividing each pulse into two pulses, the first of said twopulses being polarized orthogonally to the second of said two pulses,

means for varying the length of the path traversed by the second pulsewith respect to said first pulse so as to provide an adjustable delaybetween said two beams,

means for impinging said orthogonally polarized pulses onto said crystalsurface so as to create a sum-frequency radiation generation of said twopulses at said surface, the signal strength of the sum-frequencygeneration being a function of the degree of coincidence of said twopulses, and

detector means for measuring said sum-frequency signal strength.

References Cited UNITED STATES PATENTS 3,290,504 12/1966 Vallese et al250-217 X 3,293,438 12/4966 Davis 2502l7 X 3,326,078 6/1967 Clarke etal. 250220 X RALPH G. NILSON, Primary Examiner. T. N. GRIGSBY, AssistantExaminer.

US. Cl. X.R.

